Drinking vessels, such as cups, insulated beverage containers, canteens, and the like are used to contain fluids for drinking. Since fluids tend to spill if left in an open container, many drinking vessels include a lid. Some lids include openings for allowing controlled passage of the fluid to a user of the vessel. In some cases, the opening is a hole or slot for receiving a drinking straw. In other cases, the opening is a hole or slot for sipping directly through the lid. Various forms of such drinking vessels are known in the art. For example, paper disposable cups commonly include a plastic lid with an “X” shaped opening cut in the surface thereof for receiving a drinking straw. Many disposable coffee cups may be covered with a lid that includes a slot or spout for sipping directly through the lid. Such lids often include a secondary hole or opening for allowing passage of air through the lid, thereby equalizing the pressure inside the drinking vessel with the environment outside of the drinking vessel, as the liquid exits the vessel. Equalizing the pressure in this manner allows the fluid to more easily and smoothly exit the vessel.
Some drinking vessels are insulated to reduce thermal transfer between the fluid contained in the drinking vessel and the external environment. For example, some disposable coffee cups are formed of an insulating material, such as polymer foam. Other insulated drinking vessels may include an inner vessel and an outer shell, where at least a portion of the outer shell is spaced apart from a portion of the inner vessel. In some insulated drinking vessels, the space between the inner vessel and the outer shell is filled with air. In other cases the space may be filled with an insulating material, such as a fibrous material, a polymer foam material, or the like. In other cases, the space between the inner vessel and the outer shell is vacuum-sealed. Known drinking vessels are commonly made from paper, polymers, foam, plastic, metal, and the like.
People often use covered drinking vessels when they are on the move. For example, it may be desirable to cover a drinking vessel when transporting the fluid on foot or in a vehicle. Additionally, it may be desirable to use a covered drinking vessel in the outdoors to prevent contamination of the contained fluids by dirt, insects, or the like. It also may be desirable to cover a drinking vessel in order to further help keep its contents hot or cold, as the case may be.
As indicated above, it is known that by fully covering the vessel, its liquid contents are inhibited from pouring out through an established spout (or other opening) in the vessel due to backpressure exerted on the liquid inside the vessel. In other words, as fluid exits a covered vessel, the vacuum created by the exiting fluid causes pressure inside the vessel to drop below the pressure outside the vessel. This pressure differential causes air outside the vessel to flow through the spout and into the interior of the vessel until the pressure inside and outside the vessel reaches equilibrium. As this is taking place, the fluid inside the vessel is inhibited from exiting the vessel as quickly and/or as smoothly as it otherwise would if the internal and external pressures were the same. As indicated above, this is one reason for including a secondary hole or opening in the cover, i.e., it reduces the internal and external pressure differential and, therefore, allows the liquid to more freely escape from the vessel through the spout.
Unfortunately, prior art lids for drinking vessels do not adequately insulate the vessel's contents—whether hot or cold. In other words, lids on such vessels are a source of thermal transfer that leads to hot liquid (in the vessel) cooling too rapidly or cold liquids (in the vessel) warming too rapidly. In addition to other aspects of the design of such lids, drinking (or other) openings in the lids, as well as vents in the lids designed to equalize the internal and external pressure of the vessel, also can contribute to this undesirable heat transfer.